1. Field of the Invention
The present invention relates generally to a mobile communication system, and more particularly to a method and an apparatus for mapping transmission symbols to resources and then transmitting the symbols in a mobile communication system utilizing an Orthogonal Frequency Division Multiplexing (OFDM) scheme.
2. Description of the Related Art
Recently, for mobile communication systems, many studies have been and are being conducted on using an Orthogonal Frequency Division Multiplexing (OFDM) scheme for high-speed data transmission over wired/wireless channels. The OFDM scheme, which transmits data using multiple carriers, is a special type of a Multiple Carrier Modulation (MCM) scheme in which a serial symbol sequence is converted into parallel symbol sequences and the parallel symbol sequences are modulated with a plurality of mutually orthogonal subcarriers (or subcarrier channels) before being transmitted.
A system differentiating multiple users through the multiple sub-carriers while utilizing the OFDM scheme as a basic transmission scheme, that is, a system supporting multiple users by allocating different sub-carriers to different users, is generally called an “Orthogonal Frequency Division Multiple Access (OFDMA) scheme.”
A Hybrid Automatic Repeat reQuest (HARQ) scheme is one of the important techniques used to improve data throughput and reliability of data transmission in a packet-based mobile communication system. HARQ corresponds to a combination of the techniques of Automatic Retransmission Request (ARQ) and Forward Error Correction (FEC). According to ARQ, which is being widely used in a wired/wireless data communication system, a transmitter transmits data packets with sequence numbers attached to the data packets according to a pre-established scheme, and a receiver requests retransmission of a missing packet from among the received packets by using the sequence numbers, thereby achieving reliable data transmission.
Further, for FEC, each data packet is transmitted together with a redundant bit added thereto according to a predetermined rule, such as convolutional encoding or turbo encoding, so that the originally-transmitted data can be demodulated without noise or fading, which may occur during data transmission and reception.
In a system using HARQ, i.e., a combination of the above-described ARQ and FEC, the receiver performs a Cyclic Redundancy Check (CRC) for data demodulated through a predetermined inverse FEC process, in order to determine if the data has an error. As a result of the CRC, when the data has no error, the system using the HARQ feeds back an Acknowledgement (ACK) to the transmitter, so that the transmitter transmits a next data packet. However, when the CRC check shows that the data has an error, the HARQ system feeds back a Non-Acknowledgement (NACK) to the transmitter, so that the transmitter retransmits the previously transmitted data packet.
During this process, the receiver obtains an energy gain by combining the retransmitted packet with the previously transmitted packet. Accordingly, the HARQ system can achieve much improved performance in comparison with a typical ARQ system that does not perform such a combining process.
FIG. 1A illustrates an example of a conventional HARQ procedure. More specifically, in FIG. 1A, the horizontal axis is a time axis, and the data channel is a channel through which a data packet is transmitted.
In step 101, a transmitter initially transmits a data packet. In step 102, a receiver, having received the initially transmitted data packet, demodulates the received data packet and determines if the received data packet has an error during the demodulation. When the receiver determines that the transmitted data has not been correctly demodulated, i.e., there is an error, the receiver feeds back an NACK to the transmitter in step 102. The determination if the received data packet has an error can be achieved through a CRC check, etc.
Upon receiving the NACK, the transmitter performs a first retransmission of the data packet in step 103. However, even when the transmitter retransmits the same data as the data packet transmitted at the initial transmission in step 103, the same data may have different redundancies, i.e., different coded symbols may be transmitted. As used herein, the same data packet transmitted in steps 101, 103, and 105 is called a “sub-packet.”
Upon receiving the data packet by the first retransmission, the receiver combines the data packet of the first retransmission with the data packet of the initial transmission, according to a predetermined rule, and then demodulates the data channel using the combined data. Through a CRC of the data channel during the demodulation, when the receiver determines that the transmitted data has not been correctly demodulated, i.e., there is still an error, the receiver feeds back an NACK to the transmitter in step 104.
Upon receiving the NACK in step 104, the transmitter performs a second retransmission of the data packet in step 105, after passage of a predetermined time interval from the time point of the first retransmission. As described above, the data channels of the initial transmission in step 101, the first retransmission in step 103, and the second retransmission in step 105 carry the same data.
Upon receiving the data packet transmitted by the second retransmission 105, the receiver combines the data of the initial transmission in step 101, the data of the first retransmission in step 103, and the data of the second retransmission in step 105 with each other, according to a predetermined rule, and then demodulates the data channel using the combined data. Through this process, when the receiver determines, by CRC for the data channel, that the transmitted data has been correctly demodulated, the receiver feeds back an ACK to the transmitter in step 106.
After receiving the ACK in step 106, the transmitter transmits an initial transmission sub-packet for a next data packet together with a control channel in step 107.
FIG. 1B is a block diagram of a conventional mobile communication system for performing an HARQ operation. Referring to FIG. 1B, an encoder 111 of a transmitter 130 encodes a predetermined data packet and outputs coded symbols. A sub-packet generator 112 selects all or a part of coded symbols output from the encoder 111 at the kth transmission (k=0 to m, wherein m refers to a maximum number of retransmission times) and generates a sub-packet k including the selected symbols.
A transceiver chain 113 transmits the generated sub-packet k to a receiver 170 through a predetermined transmission and reception scheme such as an OFDM scheme.
A decoder of the receiver 170 decodes the received sub-packet k, and feeds back an ACK or NACK to the sub-packet generator 112 of the transmitter 130 according to a result of the decoding. Based on the feedback, the sub-packet generator 112 prepares and transmits a retransmission data packet (i.e. a next sub-packet) of the transmitted data packet or an initial transmission sub-packet of a new data packet.
Hereinafter, the encoder 111 and the sub-packet generator 112 for the HARQ operation will be described in more detail.
FIG. 2 illustrates a method for configuring the sub-packet by using a circular buffer in a mobile communication system.
In FIG. 2, one code block 201 indicates one data packet to be transmitted at a given time point. The encoder 202 receives the single code block 201 and outputs predetermined coded symbols 203. A total number of the coded symbols 203 output from the encoder 202 is defined by a code rate of the encoder 202, which is usually called a “mother code rate.” The total number of the output coded symbols 203 is usually a “mother code rate” as described above because partial or all coded symbols from among the output of the encoder 202 are selected for each sub-packet. Coded symbols, which are output from the encoder 202, include systematic symbols S, first parity symbols P1, and second parity symbols P2, each of which is called a “sub-block.”
A sub-block interleaver 204 interleaves the systematic symbols S, the first parity symbols P1, and the second parity symbols P2 within each sub-block. It is considered that all the interleaved symbols, which are output from the sub-block interleaver, have been stored in a circular buffer. As used herein, the name “circular buffer” is given because configuration of symbols for each sub-packet is achieved by selecting consecutive symbols on the circular buffer, and because, when a particular sub-packet passes over a last symbol point of the circular buffer, it returns to the first symbol of the circular buffer and selects next symbols for the sub-packet. That is, the transmitter configures each sub-packet by selecting partially consecutive symbols on the circular buffer, and the receiver decodes received coded symbols after mapping the received coded symbols to proper positions on the circular buffer of the same structure.
In the example of the sub-packet configuration illustrated in FIG. 2, reference numeral 206 indicates a symbol configuration for an initial transmission sub-packet, reference numeral 207 indicates a symbol configuration for a first retransmission sub-packet (or a second transmission sub-packet), and reference numeral 208 indicates a symbol configuration for a second retransmission sub-packet (or a third transmission sub-packet).
FIG. 3 illustrates Resource Blocks (RBs), each of which is a basic unit of resource allocation in a typical OFDMA system. Referring to FIG. 3, the horizontal axis is a frequency axis, and the vertical axis is a time axis.
In FIG. 3, one resource block 301 usually includes multiple OFDM symbols along the frequency axis and multiple consecutive OFDM symbols along the time axis. Although FIG. 3 illustrates 8 resource blocks in total, it is possible to use a different number of resource blocks.
FIG. 4 illustrates a conventional method in which a sub-packet configured as described above with reference to FIG. 2 is mapped to allocated resource blocks. The circular 205 in FIG. 4 is the same as the circular buffer 205 of FIG. 2. That is, the circular buffer 205 includes a systematic part indicated by reference numeral 401 and a parity part indicated by reference numeral 402. Coded symbols 206 are selected from the circular buffer, so as to configure an initial transmission sub-packet.
It is assumed that User Equipment (UE) #1 has been allocated resource blocks 0, 2, and 4 as indicated by reference numeral 407, i.e., a sub-packet 206 is transmitted through resource blocks 0, 2, and 4. Reference numeral 408 indicates a channel response on the frequency axis at a given time point. Here, the vertical axis corresponds to the intensity of a channel.
As noted from FIG. 4, resource blocks 0 and 2 are in a relatively bad channel condition, while resource block 4 is in a relatively good channel condition. According to the conventional method, the sub-packet 206 is sequentially loaded on the allocated resource blocks. Specifically, the foremost part 404 of the sub-packet 206 is carried by the resource block 0, the middle part 405 of the sub-packet 206 is carried by the resource block 2, and the rearmost part 406 of the sub-packet 206 is carried by the resource block 4.
According to the conventional mapping method, the systematic bits may be collectively carried by the resource blocks in a relatively bad channel condition, which may significantly degrade the data reception capability. Therefore, it is necessary to improve the data reception capability by preventing the systematic bits from being collectively carried by the resource blocks in a relatively bad channel environment.